The invention relates to a substrate for depositing at least one useful layer intended to be used for producing electronic, optical or optoelectronic components, as well as a method for producing such a substrate.
More particularly, the substrate according to the invention may be adapted to implement the deposition of a layer, such as epitaxy or heteroepitaxy, in a nitride-based material—e.g. GaN, AlN, InN, alloys of these compounds (AlGaN, InGaN, etc.)—by an MBE (acronym for “Molecular Beam Epitaxy”) process, or any other process using heating by infrared radiation. In particular, these types of materials are used for applications in high frequencies.
These layer deposition processes usually necessitate heating of the substrate so that its front face, arranged to nucleate the useful layer, reaches a temperature equal to a temperature determined by the deposition conditions and/or other conditions.
Thus a temperature necessary for thermal stripping the front face of the substrate, i.e. the deposition surface, may be reached.
Such thermal stripping of the front face may be necessary if contaminants are present therein or if the front face is covered by a protective layer. In the latter case, a protective layer in oxide may have initially been provided to protect the substrate from deterioration caused by extended storage and/or transport before deposition.
As an illustration, the minimum temperature necessary for thermal stripping a surface oxide layer of a substrate in Si or pSiC (polycrystalline SiC) is typically between approximately 700° C. and 1200° C., and more particularly on the order of 950° C.
This minimum temperature is on the order of 700° C. for stripping a substrate in GaN, 900° C. for a substrate in AlN, 1200° C. for a substrate in sapphire (Al2O3).
The heating source used to bring the front face of the substrate to the desired temperature is infrared radiation, which is converted into heat when it is absorbed by the substrate.
However, some substrates are not sufficiently absorbent to reach said temperature. For example, in the latter case, SiC, sapphire (Al2O3), GaN, AlN or ZnO are included.
It is thus necessary to find means of increasing the absorbency of these substrates to successfully achieve good stripping.
In addition, it is important to provide that these means implemented do not disturb the operation of the components to produce in the useful layer to deposit, such as the high-frequency operation of some components.
To increase infrared absorption while not disturbing the operation, particularly high-frequency operation, of components, providing an infrared absorbing layer placed on the rear face of the substrate, to convey heat, is known (US 2004/0152312, EP 0449 524, U.S. Pat. No. 5,296,385, WO 2006/082467). Using absorbent layers in α-Si (amorphous silicon) or in pSi (polycrystalline silicon) is thus known.
By choosing a substantial absorbent layer thickness and by providing enough infrared energy, the temperatures necessary for front face stripping may then be reached.
However, a too-thick thickness of the absorbent layer may lead to bending phenomena or even cracks in the absorbent layer or substrate if thermal (for example by different thermal coefficients of expansion) or structural (for example by different lattice parameters) incompatibilities exist between the materials used for the absorbent layer and for the substrate.
In some cases, one is thus forced to limit the thickness of the absorbent layer to prevent these phenomena so that the temperature necessary for good thermal stripping may be reached.
In addition, the infrared energy ensuring heating of the substrate may lead to excess in terms of economic costs if the energy necessary is too high and/or if furnaces operating at high power must be used.
Thus, a need exists to limit the costs of effective stripping, while guaranteeing good operation of the components to produce from the useful layer to deposit.